Trapping, selective

In their synthesis of (+)-cerulenin, Mani and Townsend employed lithiated epoxysilane 157, which they trapped with (4E,7 )-nonadienal to give a 77% yield of 158, which was further manipulated to give the natural product (Scheme 5.37) [58], as-ot, 3-Epoxy-Y,S-vinylsilanes 159 are regioselectively lithiated at the a-silyl position, and can subsequently be stereo selectively trapped with a range of electrophiles to give a-substituted epoxyvinylsilanes 160, which can in turn be isomerized to a-silyl-P-vinylketones 161 (Scheme 5.38) [59]. 

Alkoxy (R0 ) radicals react at near diffusion controlled rates with trialkyl phosphites to give phosphoranyl radicals [ROP(OR )3] that typically undergo very fast -scission to generate alkyl radicals (R ) and phosphates [OP(OR )3]. In a mechanistic study, trimethyl phosphite, P(OMe)3, has been used as an efficient and selective trap in oxiranylcarbinyl radical systems formed from haloepoxides under thermal AIBN/n-Bu3SnH conditions at about 80 °C (Scheme 27) [64]. The formation of alkenes resulting from the capture of allyloxy radicals by P(OMe)3 fulfils a prior prediction that, under conditions close to kinetic control, products of C-0 cleavage (path a. Scheme 27), not just those of C-C cleavage (path b. Scheme 27) may result. 

Enrichment of trace components This can be achieved by multiple injections into a precolumn with selective trapping and storage of the fraction of interest. Often performed with packed precolumns because of eir high saiq>le capacity and because maximum resolution is not required in the pre-sepeu ation step. 

The external subset selection trap This occurs when the selected subset for a particular subvalidation is not representative of the samples in the rest of the data. This usually leads to overly pessimistic cross-validation results, because the selected subset is outside of the space of the remaining samples. 

Small data sets (< 20 objects) + Relatively quick + Relatively quick - Can be slow, if m or number of iterations large - Selection of subsets unknown + OK, if many iterations done - Avoid using if N > 20 + Good choice. -. unless designed/DOE data - Requires time to determine/ construct cross validation array + often needed to avoid the external subset selection trap... 

Experiment (DOE) data order is randomized object order is randomized (external subset selection trap) avoid the external subset selection trap... 

Achiral A,A-diisopropyl-ferrocenecarboxamide (440) was deprotonated by Snieckus and coworkers by n-BuLi/(—)-sparteine (11) (equation 119) . The base discriminates well between the enantiotopic protons H(2) and H(5) in the substituted ring to form the diastereomer 441 with high selectivity. Trapping the intermediate with a couple of different electrophiles afforded the substitution products 442a-d with 85 to 99%... 

The bimolecular rate constant for the reaction of DPC with butadiene is determined to be 6.5 X 10 M s . Isoprene can be employed as a selective trap for triplet carbenes. Styrene is also shown to be an efficient trap for triplet carbene. (E)-p-Deutero-a-methylstyrene (89) is a very convenient reagent to diagnose the mult-plicity of the reacting carbene because it reacts with both singlet and triplet carbenes with different stereochemical outcomes. The stereochemistry of the adduct cyclopropane (90) can be easily judged by NMR (Scheme 9.28). For example, BA (22) reacts with styrene with total loss of stereochemistry, while in the reaction with dimethoxy FL (23a), the expected cyclopropane is obtained with complete retention of stereochemistry. The rate constants for the additions are (1.2 0.2) x 10 and... 

Grimalt JO, Fernandez P, Berdie L, Vilanova RM, Catalan J, Psenner R, Hofer R, Appleby PG, Rosseland BO, Lien L, Massabuau LC, Batterbee RW (2001) Selective trapping of organo-chlorine compounds in mountain lakes of temperate areas. Environ Sci Technol 35 2690-2697... 

QIT The quadrupole ion trap (QIT) utilizes a cylindrical ring and two end-cap electrodes to create a three-dimensional (3D) quadrupolar field for mass analysis. These instruments are capable of selectively trapping or ejecting ions and are often used for the sequential fragmentation and analysis experiments of product ion MS/MS. Also known as a 3D trap due to the configuration (March, 1997). 

Step 2 Hofmann elimination/Michael addition equilibrations. Acrylic acid functions as an efficient and selective trap for dimethyl amine vs. tert-butylamine, allowing for facile control of the equilibrium between 1 and 2. 

Air is passed through a cryotrap, where all organic compounds except CH4 are selectively trapped. The analytes are injected to the detector by rapid heating of the trap... 

Scheme 23 Hydroxyl radical-selective trapping reaction with y - locopheryl - iso-propyl ether (56) and y-tocopheryl ether derivatives (58, 59) showing different reactivity towards radicals...

This group has developed a new selective source of nucleophilic hydroxymethyl (CH2OH) radical by persulfate oxidation of ethylene glycol catalyzed by Ag(I) salts. This nucleophilic radical is selectively trapped by protonated N-heteroaromahc bases, providing a new general hydroxymethylation protocol (Equahon 14.12) [23]. 

The metastable, 2-D packing motif presented by some Pcs adsorbed on Ag(l 11) has also prompted the utilization of such ensembles as templates for the organization of complementary guest molecules such as fullerenes [193] or corannulenes [194], giving rise to the formation of two-component, 2-D architectures. More recently, a similar approach has been used to prepare a surface-supported, three-component system. In fact, the immersion of an Au substrate into a solution containing both a ZnPc and a Zn porphyrin led to the formation of a highly ordered, 2-D arrangement of both Pc and porphyrin which can act as a bimolecular chessboard toward the supramolecular assembly of a third component (i.e., C6o fullerene) which is selectively trapped in the open spaces (Fig. 25) [195],... 

Another disadvantage of the trap-tree system is that the trap -trees remove large numbers of parasites and predators of the bark beetle. By selectively trapping bark beetles in pheromone baited traps, the ratio of natural enemies to bark beetles is increased. It is probable that this altered survivorship in turn may result in higher bark beetle mortality rates. 

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